We did this a few months ago - I would suggest about 7am or so. You don't have to get on the monorail, but between waiting for the bus and waiting for MK to open for ADRs, you want some cushion. The good news is that there aren't many people fighting for the busses at that hour.
My favorite part is being let in to walk up mainstreet with hardly anyone around. Soak it in and enjoy!